1. Field of the Invention
The present invention relates to an apparatus for developing electrostatic latent images.
2. Description of the Prior Art
Various electrophotographic systems have been known and are being proposed for developing electrostatic latent images on latent image carriers into visible images.
According to one known electrostatic latent image developing system, one-component toner, i.e., a powder developer comprising only toner without carrier particles, is placed as a thin layer on a toner carrier and moved into a developing section by rotating the toner carrier, so that an electrostatic latent image can be developed by the toner. For good image development, the thin toner layer on the toner carrier must be uniform in thickness and electrically charged uniformly. To hold the toner as a thin layer on the toner carrier, a member for regulating a toner layer thickness is resiliently pressed against the toner carrier through the toner. As time goes on, however, the toner tends to be attached to the toner layer regulating member, and surface irregularities are formed by the attached toner on the contact portion or edge of the toner layer regulating member which contacts the toner layer surface to make the same even, thus failing to form a toner layer having a uniform thickness. The surface of the toner layer which is contacted by the toner layer regulating member with irregular toner deposits thereon then has small grooves and ridges. The grooves will appear as white stripes and the ridges as black stripes on a developed visible image, which greatly impair the quality of the image.
To prevent the toner from being attached as irregular toner deposits to the toner layer regulating member, it has been proposed to form the edge of the toner layer regulating member, which is held against the toner carrier through the toner layer, of a material that can easily be worn by abrasive contact with the toner. The idea is that the edge of the toner layer regulating member would be worn by frictional contact with the toner while forming a thin layer thereof on the toner carrier, and no toner would be deposited on the edge of the toner layer regulating member.
However, white and black stripes are nevertheless formed by the toner layer regulating member with the easily wearable edge. At the time such white and black stripes are produced, the toner layer regulating member is checked and it is found that toner has been deposited on the easily wearable edge of the toner layer regulating member. Before such toner deposits are formed, it is also found that the toner layer regulated by the toner layer regulating member is irregular in thickness.
One latent image developing apparatus that has heretofore been proposed comprises a photosensitive drum serving as a latent image carrier, a toner container of one-component toner, and a drum-shaped toner carrier disposed between the photosensitive drum and the toner container. The apparatus also includes a toner layer regulating member in the form of a thin plate having at least a distal edge formed of an easily wearable material, the toner layer regulating member being pressed against the toner carrier under spring forces. As the toner carrier rotates about its own axis, toner placed thereon is formed into a thin layer by the toner layer regulating member, and at the same time electrically charged to such a polarity and extent that are required for latent image development. The toner layer is supplied and electrostatically attracted to the photosensitive drum to visualize the latent image thereon.
The toner layer regulating member with the easily wearable edge has however proven unsatisfactory for the reason described above.
Where the toner layer regulating member is made of metal, it is advantageous in that it has a relatively intermediate charging capability with respect to toner of a polarity (-) and toner of a polarity (+), and can be used with both polarities, and that a suitable metallic material of low hardness and easy wearability can be used. However, since metal is generally poor in resiliency, it is difficult for the metallic toner layer regulating member to form a uniform thin toner layer on the toner carrier, unless the toner layer regulating member is kept highly accurate dimensionally with respect to the toner carrier.
In case the toner layer regulating member is made of synthetic resin, it is resilient and hence free of the above shortcoming. However, inasmuch as the charging capability of synthetic resin is generally for the polarity (+) or (-), it may sometimes fail to meet toner of a certain polarity, and cannot be used with respect to both toner polarities.
Therefore, the toner layer regulating member made of metal only and the toner layer regulating member made of synthetic resin only have their own advantages and disadvantages.